In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. The theoretically possible maximum NA is 1.35. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Non-Patent Document 1).
In conjunction with the immersion lithography, several problems arising from the presence of water on the resist film are pointed out. For example, since the resist film is in contact with water, a photoacid generator in the resist composition, an acid generated therefrom upon exposure, and an amine compound added to the resist film as a quencher can be leached in water. Such leaching causes changes of the pattern profile and contamination of the projection lens of the exposure tool.
The problem may be solved by forming a protective film for restraining leaching. Although the protective film is effective for restraining leaching, another problem arises that the protective film adversely affects pattern profile and resolution. The problem pertaining to pattern profile is that the pattern is rounded at the top, known as top-loss.
As the demand for high resolution increases, further improvements in lithography properties are desired. Among others, the pattern formation process needs further improvements in depth of focus (DOF) and line width roughness (LWR) for gaining a wider process margin.
The resist material adapted for the ArF excimer laser lithography must have satisfactory resist properties including transparency at wavelength 193 nm, sensitivity, resolution, dry etching resistance, and pattern profile. As the resist material having satisfactory fundamental properties, Patent Document 1 proposes a resist composition comprising a base resin in the form of a poly(meth)acrylic acid derivative having a bulky acid-decomposable protective group (typically 2-ethyl-2-adamantyl or 2-methyl-2-adamantyl) and a monocyclic acid-decomposable protective group capable of effective removal (typically 1-methylcyclopentyl or 1-methylcyclohexyl) and a sulfonium salt of naphthalene skeleton (typically 1-(4-n-butoxynaphthyl)tetrahydrothiophenium nonafluoro-n-butanesulfonate). This resist composition may be adjusted in sensitivity and dry etching resistance to some extent by changing the ratio of the acid-dissociable group of adamantyl structure to the acid-dissociable group of cyclopentyl or cyclohexyl structure. However, since the sulfonium salt of nonafluoro-n-butanesulfonate has a low acid diffusion controlling ability, the resist composition is not sufficient with respect to the DOF of removal (grouped pattern) performance and retention (isolated pattern) performance. Specifically, when the ratio of the acid-dissociable group of adamantyl structure is increased, the retention performance is secured, but the removal performance and LWR are degraded. When the ratio of the acid-dissociable group of cyclopentyl or cyclohexyl structure is increased, the removal performance is secured, but the retention performance and pattern profile are degraded. That is, the resist composition is not satisfactory when the trend toward further miniaturization is considered. In the immersion lithography, the sulfonium salt of nonafluoro-n-butanesulfonate is leached out more, which is also a problem.